Some general liability and/or umbrella policies, at least in connection with a civil suit, might cover costs of defense and indemnify for any damages awarded. And I believe that the NRA has made arrangements for some type of insurance coverage to be available to members.
If you have substantial personal assets, you probably want to look into those kinds of insurance.
"It is long been a principle of ours that one is no more armed because he has possession of a firearm than he is a musician because he owns a piano. There is no point in having a gun if you are not capable of using it skillfully." -- Jeff Cooper